1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a closed loop cooling circuit for the turbine of an industrial gas turbine engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine includes a compressor to compress air that is passed into a combustor and burned with a fuel to produce a hot gas flow. This hot gas flow is then passed through a turbine that typically includes three or four stages of stator vanes and rotor blades to extract energy from the hot gas flow to drive the compressor and, in the case of an industrial gas turbine (IGT) engine, an electric generator.
The efficiency of the engine can be increased by passing a higher temperature gas flow into the turbine. A typical turbine inlet temperature for an engine is 3,000 degrees F. which is high enough to melt the first stage airfoils. To allow for the turbine parts to withstand this extreme temperature environment, the airfoils are cooled by passing cooling air through a complex cooling circuit formed within the airfoils. The compressed air passed through the airfoils for cooling is bled off from the compressor. Around 20% of the compressor outlet is used for cooling of various turbine airfoils. Since the cooling air does not perform any work in the engine, this is lost energy.
It is known to bleed a portion of the compressed air produced in the compressor section of a gas turbine engine for use as a cooling medium in the turbine portion of the engine. The compressed air may be injected into the flow of combustion gas to provide an insulating film over the turbine surfaces, or it may be passed through internal cooling passages formed in the hot turbine parts in a closed cooling system. One example of such a prior art device is illustrated in U.S. Pat. No. 5,782,076 issued to Huber et al. on Jul. 21, 1998, incorporated by reference herein.
A closed loop cooling system is used to improve the cooling effectiveness of the engine so that compressed air from the compressor is not required. U.S. Pat. No. 6,295,803 issued to Bancalari on Oct. 2, 2001 (incorporated herein by reference) discloses a gas turbine engine having a closed-loop cooling circuit for transferring heat from the hot turbine section to the compressed air produced by the compressor section. The closed-loop cooling system includes a heat exchanger disposed in the flow path of the compressed air between the outlet of the compressor section and the inlet of the combustor. A cooling fluid may be driven by a pump located outside of the engine casing or a pump mounted on the rotor shaft. The cooling circuit may include an orifice for causing the cooling fluid to change from a liquid state to a gaseous state, thereby increasing the heat transfer capacity of the cooling circuit. The Bancalari patent discloses that the cooling fluid can be steam, air, glycol, liquid metals or other cooling medium.